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(KMOV.com) -- A letter sent to an interracial couple in Jefferson County has police officers on alert.

The letter, which used racist language and carried a vague threat of violence, was hand delivered the day after Christmas to the family’s home in Crystal City.

A knock on the door at 2 p.m. drew the couple’s attention, but when they opened the door there was only the message waiting in their mailbox, sent from The Sons of Liberty. The letter said the group has pledged to take up arms against all enemies of the United Sates and, in its most offensive language, says that includes:

“…rag-heads, mixed breeders and their offspring.” It goes on to say the group, “will strike our enemies when the day comes.”

There is a militia group called Sons of Liberty with chapters in Illinois, but police in Crystal City say they have never heard of anyone or any group using that name in the area.